The present invention relates in general to data communications and, more particularly, to a circuit and method of biasing a drive transistor to a data bus.
A data bus may comprise one or more electrical conductors that allow communications between a first transmitting and receiving unit (transceiver) and a second transceiver. In automotive applications, the data bus is typically a single conductor that serially transfers commands and status information regarding the vehicle's operation. Many of the automobile functions that were previously controlled with dedicated hard-wire connections are now digitally encoded in one transceiver and transferred over the data bus to another transceiver. For example, when the driver steps on the brake pedal, a control transceiver senses the action and sends a "brake light illuminate" command as a serial 8-bit instruction along the single conductor to the rear of the vehicle where another transceiver receives the serial 8-bit instruction and turns on the brake lights.
In another example, automobiles with air bags have a detonation device, commonly called a squib, that inflates the air bag upon sensing a collision. It is important for the occupant's safety, and the auto manufacturer's liability, that the squib fire at the appropriate time. Hence, the squib is regularly monitored to check its operational status. A control transceiver sends a status request over the data bus to the squib IC transceiver. The squib IC performs the status check and returns the status information back over the data bus to the requesting control transceiver that illuminates a warning light should the squib fail status check.
Thus, the automobile has multiple transceivers that take control of the data bus when it is free to transmit information. There are also multiple transceivers that read from the data bus. The transmission of data must meet industry standard specifications regarding data rate, rise and fall times, and corner transitions of the waveforms. The transceivers are each referenced to chassis ground at the location that it is mounted to the vehicle. Unfortunately, chassis ground can differ by as much as .+-.2.0 volts around the vehicle. The difference in ground potential may cause one transceiver to hold the data bus at a voltage level higher than expected by another transceiver. The higher voltage on the data bus can cause sharp transitions of the data signal, especially on the corner and rising edge of the waveform, in excess of the industry standards. The sharp transitions and associated high frequency components on a long conductor data bus act as an antenna and cause radiated emissions that can interfere with other electronics in the automobile.
Hence, a need exists to maintain the industry standard data rate and rise and fall times and corners of the transmitted waveforms and avoid radiated emissions.